This command is used in order to manage the consumer groups associated
with a stream data structure. Using
XGROUP you can:
- Create a new consumer group associated with a stream.
- Destroy a consumer group.
- Remove a specific consumer from a consumer group.
- Set the consumer group last delivered ID to something else.
To create a new consumer group, use the following form:
XGROUP CREATE mystream consumer-group-name $
The last argument is the ID of the last item in the stream to consider already delivered. In the above case we used the special ID ‘$’ (that means: the ID of the last item in the stream). In this case the consumers fetching data from that consumer group will only see new elements arriving in the stream.
If instead you want consumers to fetch the whole stream history, use zero as the starting ID for the consumer group:
XGROUP CREATE mystream consumer-group-name 0
Of course it is also possible to use any other valid ID. If the specified
consumer group already exists, the command returns a
Otherwise the operation is performed and OK is returned. There are no hard
limits to the number of consumer groups you can associate to a given stream.
A consumer can be destroyed completely by using the following form:
XGROUP DESTROY mystream some-consumer-group
The consumer group will be destroyed even if there are active consumers and pending messages, so make sure to call this command only when really needed.
To just remove a given consumer from a consumer group, the following form is used:
XGROUP DELCONSUMER mystream consumergrouo myconsumer123
Consumers in a consumer group are auto-created every time a new consumer name is mentioned by some command. However sometimes it may be useful to remove old consumers since they are no longer used. This form returns the number of pending messages that the consumer had before it was deleted.
Finally it possible to set the next message to deliver using the
SETID subcommand. Normally the next ID is set when the consumer is
created, as the last argument of
XGROUP CREATE. However using this form
the next ID can be modified later without deleting and creating the consumer
group again. For instance if you want the consumers in a consumer group
to re-process all the messages in a stream, you may want to set its next
ID to 0:
XGROUP SETID mystream my-consumer-group 0
Finally to get some help if you don’t remember the syntax, use the HELP subcommand: